When you use the lasso tool to make a selection the drawing tools and eraser tool will only work within the selected area. In order to draw outside this area you first need to release the selection, as Michele points out in her message. To release a selection you can go to the Select menu and choose Deselect. Michele has also included the keyboard shortcut for this.
The good thing about a selection is that it helps you to prevent drawing over areas that you want to protect. Just select the area that you want to draw inside and you should be able to draw inside the selection but not outside of it. Once you have finished drawing, or you want to draw in another area, release the selection.
Hope this helps,
Angie Taylor | Creative Cabin